In the case of subjecting a to-be-emulsified material to O/W emulsification, it is necessary to use an organic solvent capable of dissolving the to-be-emulsified material therein to dissolve it. Also, in order to obtain a stable emulsion, it is necessary to select an appropriate surfactant as an emulsifying agent, taking into consideration physicochemical characteristics of an organic solvent solution of the to-be-emulsified material, and carry out emulsification.
For example, JP-A-3-50279 discloses an aqueous release agent which is characterized by comprising an aqueous dispersion of a long-chain alkyl graft polymer obtained by mixing and stirring an aqueous solution of a polyamine and an alkyl isocyanate in the presence of hexane, ethyl acetate or toluene, thereby allowing the mixture to react. Also, JP-A-11-172225 discloses a release agent obtained by emulsifying a reaction product of EVA and an alkyl isocyanate having from 8 to 30 carbon atoms with a petroleum based solvent or decyl alcohol and with a surfactant which is a combination of a surfactant having a high HLB and a surfactant having a low HLB. Also, JP-A-2002-363289 discloses a method for producing a polymer aqueous dispersion by emulsifying and dispersing a mixture of a dispersoid dissolved in toluene and a dispersion medium by injection under an injection pressure of 6.5×107 Pa or more. Furthermore, JP-A-2002-129031 discloses a polymer aqueous dispersion and a method for producing the same, in which a releasable polymer having a long-chain alkyl group having 8 or more carbon atoms is dissolved in toluene, and the solution is emulsified and dispersed in water using an emulsifying agent having at least one long-chain hydrocarbon group having 15 or more carbon atoms. Further, JP-A-2003-221448 discloses a method for producing a polymer aqueous dispersion, which is characterized by emulsifying and dispersing, in water, a toluene solution of a dispersoid containing a releasable polymer having a long-chain alkyl group having 8 or more carbon atoms, by an emulsifying agent which is a combination system of an ampholytic surfactant and a nonionic surfactant, to form a microemulsion having a volume average particle size of not more than 0.1 μm.
However, with respect to a to-be-emulsified material which is sparingly soluble in an organic solvent, or the like, it is difficult to prepare a stable emulsion thereof, and as the one and only method, there is a melt emulsification method for emulsifying a to-be-emulsified material together with a surfactant at a high temperature of the melting point of the to-be-emulsified material or higher. For example, JP-A-11-70733 discloses a method in which a color-developing agent is heated at the melting point thereof or higher in an airtight container to achieve emulsification, followed by coating the emulsion. It is necessary to note that diethyl maleate which is used in all of the working examples is actually able to function as an organic solvent that dissolves the color-developing agent. Also, in Example 2 only in which the emulsification is performed under a high-temperature condition as it is, the emulsification is performed under a high-temperature and high-pressure condition by a dissolver using a general surfactant such as sodium dodecylbenzenesulfonate and sodium lauryl sulfate. However, with such a simple stirring and mixing condition as the said condition, the surfactant foams by the highly rotating dissolver, so that it does not work for the emulsification of the to-be-emulsified material, making it impossible to stably form an emulsion. Thus, a doubt remains in its results. Also, JP-A-2001-55302 discloses a formation method in which a solid is continuously melted, a surfactant or a dispersant is added thereto, and the mixture is suspended in an airtight chamber under a high shear condition. However, prior to the shearing, a solvent flow of such a low temperature that crystallizes the solid in a molten state during retention within the chamber is made joined together. It cannot be considered that the emulsification is achieved in a state where the solid under a high temperature is thoroughly dissolved, and it is difficult to stably obtain an emulsion having a small particle size regardless of the kind of the to-be-emulsified material.
In this melt emulsification method, a high-temperature treatment at the melting point of the to-be-emulsified material or higher is generally required. In the case of high-melting point materials to be emulsified, the treatment temperature greatly exceeds 100° C., at which water boils at normal pressure. Thus, it is necessary to restrain boiling of water by means of considerable pressurization. It becomes necessary a method for producing an O/W emulsion using a dispersion method capable of stably performing emulsification under a high-temperature and high-pressure condition using an emulsifying agent which even when such a high-temperature and high-pressure treatment is performed, is able to form a stable emulsion without causing decomposition or the like, while avoiding physical disturbance. However, most of emulsifying agents have a phase transition temperature of from about 60 to 80° C. and a melting point of from about 50 to 60° C. at the utmost, and only methods for producing an O/W emulsion capable of forming a stable emulsion at such a temperature level or lower have been reported. With respect to the dispersion method, a production method of stably forming an O/W emulsion by combining a melted to-be-emulsified material with an emulsifying agent whose phase transition temperature or melting point is high has not been known yet. In the fields of pharmaceutical agents, cosmetics, resin materials and the like, aqueous preparations are desired from the standpoints of environmental responsiveness and safety, and a method for producing an emulsion substantially eliminating an organic solvent is demanded.
Also, in conventional emulsions, when moisture which functions as a dispersion medium vanishes, emulsion particles are fused each other. Therefore, the resulting product cannot be made into powder by drying or the like. For that reason, the coexistence with a large amount of water functioning as a dispersion medium is essential, and there are such problems that the weight becomes large unavoidably, and that with materials to be emulsified that are susceptible to hydrolysis, a problem arises in long-term storage. If an emulsified material can be dried and made into powder as it is and can be easily restored to an emulsion at the time of use as the case may be, such is advantageous in terms of transportation cost and long-term storage. Such an emulsion has been demanded.
Also, as examples of using an organic nanotube, JP-A-2004-261885 discloses a method for introducing a functional material into an organic nanotube composed of sugar and a hydrocarbon; and U.S. Pat. No. 5,492,696 discloses a method in which after freeze-drying a nanotube formed of a phospholipid, a functional material is introduced into the nanotube, thereby performing release control.